Add: Reference for `collecting functions`
Add: Reference for `collecting functions`

Trying to fix the doc build which was broken around the time this new
section was added. Build errors with <unknown> key from
`_plugins/references.rb`

nickanderson/documentation-generator,stweil/documentation-generator,nickanderson/documentation-generator,cfengine/documentation-generator,michaelclelland/documentation-generator,michaelclelland/documentation-generator,michaelclelland/documentation-generator,cfengine/documentation-generator,stweil/documentation-generator,michaelclelland/documentation-generator,stweil/documentation-generator,cfengine/documentation-generator,cfengine/documentation-generator,nickanderson/documentation-generator,michaelclelland/documentation-generator,nickanderson/documentation-generator,stweil/documentation-generator,cfengine/documentation-generator,nickanderson/documentation-generator,stweil/documentation-generator